Thermal Degradation Mechanisms of
Polybenzoxazines
Jale Hacalog˘lu,*,1Tamer Uyar,{and Hatsuo Ishida{
*Department of Chemistry, Middle East Technical University, Ankara, 06531, Turkey
{UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara, 06800, Turkey
{Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio, 44106, USA 1
Corresponding author: E-mail:jale@metu.edu.tr
1. INTRODUCTION
Among the several techniques used to investigate the thermal characteristics of polymers, thermogravimetric analyzer (TGA), TGA interfaced with Fourier transform infrared spectroscopy (FTIR) or gas chromatography-mass spectrometry (GC-MS), and pyrolysis techniques coupled with FTIR, GC-MS, and MS provide information on thermal degradation products [1–11]. FTIR combined with TGA or a pyrolysis technique provides information about the type and functionality of the degradation products as a function of time and/or temperature [1,2]. Yet, informa-tion on neither the exact structure of all the decomposiinforma-tion products nor the separation of different decomposition products coming off at the same time can be provided. The use of GC-MS instead of FTIR allows the separation and identification of degradation products [4,7]. However, condensation of high mass pyrolysates and reactions between the decomposition products during the transport of degradation products from TGA or pyrolyzer system to FTIR or GC-MS are highly probable. With the direct pyrolysis-mass spectrometry (DP-MS) technique, second-ary condensation reactions are eliminated and the detection of primary degradation products, high mass pyrolysates, and unstable thermal degradation products becomes possi-ble as a consequence of removal of the thermal degradation products rapidly from the heating zone by the high vacuum system and rapid detection system [10,11]. Thus, a better un-derstanding of the thermal characteristics, polymerization, and cross-linking processes can be achieved.
The thermal degradation processes of various polyben-zoxazines were studied, and the degradation mechanisms were proposed in the literature [12–22]. The structural effects of phenols and amines on the thermal degradation of polybenzoxazines were investigated systematically
[12–17,20]. The thermal decomposition studies of polybenzoxazines are sometimes difficult because a large number of degradation products are released during the pyrolysis of polybenzoxazines; therefore, model dimers and oligomers of polybenzoxazines were also examined in order to have a clear understanding of the thermal degra-dation mechanisms of polybenzoxazines [12,18,19,21]. Here, the findings related to the thermal degradation processes of polybenzoxazines and the proposed degrada-tion mechanisms for polybenzoxazines are summarized.
2. THERMAL CHARACTERISTICS OF
POLYBENZOXAZINES
Benzoxazines polymerize via a thermally activated cationic ring-opening reaction to form a phenolic structure character-ized by a Mannich-based bridge (CH2NRCH2), as
shown in Scheme 1. The chemical structures of the most common benzoxazines such as phenol-methylamine-based (Ph-m), phenol-aniline-based (Ph-a), bisphenol A-methyl-amine-based (BA-m), and bisphenol A-aniline-based (BA-a) benzoxazines are given inScheme 2. The thermal degradation processes of polybenzoxazines were studied by TGA, TGA interfaced with a Fourier transform infrared spectrometer (TGA-FTIR), evolved gas analysis (EGA) performed by GC-MS, and DP-MS [12–22]. OH N R N O R OH OH N R N O R n OH
SCHEME 1 Benzoxazine polymerization.
Handbook of Benzoxazine Resins. DOI: 10.1016/B978-0-444-53790-4.00059-X
2.1. Thermogravimetric Analysis of
Polybenzoxazines
The TGA thermograms of poly(Ph-m), poly(Ph-a), poly (BA-m), and poly(BA-a) are given in Figures 1–4, respectively. The TGA thermograms show that more than one degradation step occurs during the thermal degra-dation of polybenzoxazines. In TGA thermograms,
polybenzoxazines show an initial weight loss starting around 260-280 C and the main degradation occurs between 300 and 450C depending on the molecular struc-ture of polybenzoxazines [12,13,20]. As can be seen from the derivative of weight loss of polybenzoxazines in Figure 1–4, multiple weight loss events occur during the heating from room temperature to 800 C. The initial weight loss at the low temperature region (around 260-280C) is due to the evaporation of amines and the ma-jor weight loss at temperatures between 300 and 450C is due to the degradation of the phenolic moieties. These find-ings are supported by evolved gas analyses (EGAs) by FTIR and GC-MS because TGA by itself does not provide enough information about the thermal degradation products of polybenzoxazines [12,13,20]. Hence, evolved gas analyses (EGA) need to be carried out by FTIR and GC-MS in order to investigate the degradation products and the thermal degradation mechanism of polybenzoxazines.
2.2. Evolved Gas Analyses (EGA) of
Polybenzoxazines by FTIR
The FTIR evolved gas analysis (EGA) for aromatic amine-based and aliphatic amine-amine-based polybenzoxazines has been carried out [12,13]. It is worth mentioning that exact identification of the degraded products is difficult from the FTIR spectra alone, because the FTIR spectra are of mixtures of the degraded compounds; however, certain characteristic absorption bands are good enough for the classification of the particular species. The FTIR evolved gas analyses (EGA) of polybenzoxazines were divided into three sections: (1) the initial degradation stage starting Ph-m BA-a BA-m Ph-a O N O N O N H3C CH3 CH3 O O N O N N H3C O N O N O N H3C CH3 CH3 CH3 CH3 O O N O N N H3C H3C
SCHEME 2 The chemical structures of phenol-methyl-based (Ph-m), phenol-aniline-based (Ph-a), bisphenol A-methyl-based (BA-m), bisphe-nol A-aniline-based (BA-a) benzoxazine monomers.
from room temperature to 300 C, (2) the intermediate degradation stage between 300 and 400C, and (3) the final degradation stage, which occurs above 400C. In the case of aliphatic amine-based polybenzoxazines, the initial degradation products were aliphatic amines while aniline was a major degradation product in aromatic amine-based polybenzoxazines. At the intermediate degradation stage, substituted phenols were evolved. At the final degradation stage, which occurred above 400 C, only phenols were detected.
Low and Ishida have carried out systematic FTIR evolved gas analyses for aliphatic amine-based polybenzoxazines, namely, bisphenol A methylamine-based (BA-m), bisphenol A ethylamine-methylamine-based (BA-e), bisphenol An-propylamine-based (BA-np), and bisphenol A amylamine-based (BA-amyl) polybenzoxazines [12]. For all aliphatic amine-based polybenzoxazine compounds, aliphatic amines were detected at the initial degradation stage (below 300C). It was observed that the initial deg-radation step for poly(BA-m) and poly(BA-e) differed FIGURE 2 TGA thermogram and its derivative from the degradation of poly(Ph-a) under nitrogen.
significantly from that for poly(BA-np) and poly(BA-amyl). The initial degradation products were dimethylamine and methylethylamine for poly(BA-m) and poly(BA-e), respec-tively. In the case of poly(BA-np) and poly(BA-amyl), the release of ammonia was detected, which indicates that the Mannich base and the substituent attached to the nitrogen must have cleaved. In addition, the release of various amines, possibly secondary or tertiary amines, were also detected at the initial degradation stage. Comparison of the spectra obtained from the initial degradation step of polybenzoxa-zines with the vapor-phase spectra of methylamine, ethyla-mine, propylaethyla-mine, and amylamine confirmed that primary amines are not major degradation components for these ali-phatic amine-based polybenzoxazines. For poly(BA-m), a strong band at 1670 cm1 was detected at around 280C, which was assigned to a free Schiff base (C¼¼N). At this ini-tial stage, no aromatic band was detected in the FTIR spec-trum; thus, aliphatic species and aliphatic Schiff bases are most likely the decomposition products. In the case of poly (BA-e), poly(BA-np), and poly(BA-amyl), the observed Schiff base band was not as strong as in poly(BA-m), indi-cating that the concentration of the Schiff base may be depen-dent on the type of amine substituent.
At the intermediate degradation stage (between 300 and 400C) of aliphatic amine-based polybenzoxazines, no al-iphatic amine absorption bands were observed in the FTIR spectra of evolved gas analyses (EGA), indicating that the amines that evaporated very likely come from the amines that are part of the branches and chain ends. In addition, the concentration of the Schiff base was also decreased at this stage. At around 400 C, a sharp peak at 3650 cm1 was detected in the FTIR spectra of all the aliphatic amine-based polybenzoxazines. This band is due to the free
OH stretching. In addition, substituted benzene modes at 1600, 1480, and 747 cm1were also detected in the spectra. These findings suggested that substituted phenols are the degradation products at this stage. The type of substituted phenols is difficult to identify by FTIR spectra alone, yet possible candidates would be phenol, p-cresol, o-cresol, dimethylphenol, trimethylphenol, isopropyl-phenol, etc. Above 400C, the recorded FTIR spectra of the evolved gas of degraded polybenzoxazines were similar to each other. It was observed that the characteristic band of the aliphatic amine had disappeared totally and only various substituted phenols were detected.
The FTIR evolved gas analyses for aromatic amine-based polybenzoxazines were also studied [13]. In this case, aniline was the major degradation product at the initial degradation stage. The evaporation of amine is a conse-quence of the Mannich base cleavage. The Schiff base was also detected as a degradation product for aromatic amine-based polybenzoxazines. At around 400C, a signif-icant band at 3650 cm1was detected from degraded poly (BA-a). This band is due to the free OH group of phenol and it is accompanied by a band at 1180 cm1, which is due to the CO bond of phenol or substituted phenols. This temperature corresponds to the temperature of the maxi-mum rate of weight loss in the TGA thermogram of poly (BA-a). Thus, the main degradation temperature observed in TGA can be assigned to phenolic cleavage for poly (BA-a).
In short, the TGA and FTIR evolved gas analyses (EGA) indicate that multiple processes are occurring in thermal decomposition of polybenzoxazines. The degradation products are mainly various types of aliphatic or aromatic amines, aliphatic or aromatic Schiff bases, and substituted FIGURE 4 TGA thermogram and its derivative from the degradation of poly(BA-a) under nitrogen.
phenolic compounds, depending on the corresponding ben-zoxazine monomer structure. Based on the TGA and FTIR evolved gas analyses (EGA), the thermal degradation mechanisms of polybenzoxazines have been proposed [12]. Polybenzoxazines have both inter- and intramolecular hydrogen bonding [23] and the presence of the hydrogen bonding is expected to affect the degradation mechanism. For instance, the hydrogen bonding between the OH group and the N of the Mannich base results in a conformationally preferred six-membered ring. The thermal cleavage of the Mannich base in the presence of this hydrogen bond is pro-posed inScheme 3. The cleavage of the CN bond that is not part of the six-membered ring is more likely to occur because the six-membered ring is energetically more stable. In addition, the cleavage of the CN bond rather than the CC bond from the Mannich base is also expected as the CC bond (bond energy ¼ 82.6 Kcal/mol) has higher dis-sociation energy than the CN bond (bond energy ¼ 72 Kcal/mol) [24]. A Schiff base (C¼¼N) is formed when the Mannich base is cleaved. So, the next likely cleavage point is the CC bond as the CC bond (bond energy ¼ 82.6 Kcal/mol) has a much lower bond energy than a C¼¼N bond (bond energy ¼ 147 Kcal/mol) [24]. Thus, an aliphatic Schiff base is the degradation product resulting from this cleavage. However, in the case of bisphenol A-based polybenzoxazines, the aromatic Schiff base is also likely to be produced as a degradation product as the bisphe-nol A backbone is not thermally stable and therefore cleav-age of the isopropyl link in the bisphenol A would yield an aromatic Schiff base.
In the aromatic amine-based polybenzoxazine structure, there are two equivalent CN bonds that have equal
probability of thermal cleavage in the absence of intramolecular hydrogen bonds. In the case of aliphatic amine-based polybenzoxazines, three CN bonds are present as possible cleavage points. This explains the detec-tion of ammonia as a degradadetec-tion product for all the ali-phatic amine-based polybenzoxazines. The proposed degradation mechanism is shown inScheme 4. This degra-dation route provides ammonia and a secondary amine, both of which are detected from the evolved gas analyses of al-iphatic amine-based polybenzoxazines. In the case of aro-matic amine-based polybenzoxazines, aniline is evolved as a degradation product. It is proposed that the degradation route shown inScheme 4is favored at lower temperatures than the degradation route shown inScheme 3as the pres-ence of the intramolecular hydrogen bonding has a ten-dency to stabilize the polybenzoxazine.
2.3. Evolved Gas Analyses (EGA) of
Polybenzoxazines by GC-MS
The evolved gas analyses (EGA) by using GC-MS were performed for determination of the thermal decomposition products of polybenzoxazines [20]. In GC-MS, the thermal decomposition products are a direct result of either the degradation of the polybenzoxazine or the recombination of the degradation compounds formed during the pyrolysis process. The degradation products of polybenzoxazines identified by GC-MS are grouped into eight categories as follows: benzene derivatives, amines, phenolic com-pounds, Mannich base comcom-pounds, 2,3-benzofuran deriva-tives, iso-quinoline derivaderiva-tives, biphenyl compounds, and phenanthridine derivatives. Primary degradation products
H3C CH3 OH H3C OH H3C CH3 OH H3C OH H3C CH3 OH OH N R CH3 OH OH OH OH H3C R N H O H3C H3C H3C H3C CH3 CH3 OH H3C OH CH3 OH OH CH3 CH3 CH3 H3C OH OH N R OH OH H2C OH OH OH H3C H3C R N H O N R
SCHEME 3 Proposed Mannich base cleavage in the presence of an intramolecular hydrogen bond.
of polybenzoxazines are benzene derivatives, amines, phenolic compounds, Mannich base compounds, while 2,3-benzofuran derivatives, iso-quinoline derivatives, biphenyl compounds, and phenanthridine derivatives are secondary degradation products. The compositions of the primary degradation products of polybenzoxazines are given below.
2.3.1. Benzene Derivatives
The benzene derivative compounds detected by GC-MS are toluene, dimethylbenzene, and trimethylbenzene. For poly (BA-a) and poly(Ph-a), m-xylene and 1,3,5-trimethylben-zene compounds have the highest intensities. In the case of poly(BA-a), m-xylene and 1,3,5-trimethylbenzene are the degradation products that can result directly from the degradation of the polymers as poly(BA-a) has a 2,4,6-tri-substituted phenolic structure. However, for poly (Ph-a), the presence of these two degradation products is detected despite the absence of the 1,3,5-trimethylbenzene type structure in the Ph-a monomer. Ph-a has both free ortho and free para positions, which can be subjected to
the C-aminoalkylation reaction during the ring-opening polymerization. Therefore, the 2,4,6-tri-substituted phenolic structure is also expected to be present in the network structure of the poly(Ph-a), as shown inScheme 5.
2.3.2. Amines
The major portion of the amines resulting as degradation products for polybenzoxazines is the original amine reactant that is used to obtain benzoxazine monomer. For instance, aniline is one of the major degradation products for both poly(BA-a) and poly(Ph-a). Besides primary amines, substituted amines are also detected as the decomposition products. This suggests that theortho and para positions of the aniline are also contributing as the reactive side for the polymerization reaction [17].
2.3.3. Phenolic Compounds
The majority of the phenolic compounds detected by GC-MS for polybenzoxazines are phenol and substituted phenols such as mono- and dimethylphenol. For poly (BA-a), p-iso-propyl phenols were also detected. Phenolic H3C CH3 OH OH H3C CH3 OH OH N R H3C CH3 OH OH CH3 H3C CH3 OH OH H3C NH2 R H3C OH OH OH OH N R H3C OH OH CH3 OH OH H3C NH2 R H3C CH3 OH OH H3C CH3 OH OH N R H3C CH3 OH OH N R CH3 H3C H2C CH3 OH OH H3C CH3 OH OH CH3 H3C CH3 OH OH NH R H3C H3C OH OH H3C OH OH N R H3C CH3 OH OH N R CH3 H3C H2C OH OH H3C CH3 OH OH CH3 H3C CH3 OH OH OR NH R H3C CH3 CH3 CH3 CH3 CH3 H3C CH3 H3C CH3
SCHEME 4 Proposed Mannich base cleavage in the absence of a intramolecular hydrogen bond.
compounds are the direct result of polybenzoxazine degradation. The studies on the decomposition of the polybenzoxazine model dimers and oligomers have also reported phenolic compounds as one of the major degradation products [18,21].
2.3.4. Mannich Base Compounds
The Mannich base compounds detected by GC-MS are obviously direct results of the polybenzoxazine degradation as well. As discussed in the previous section, two fragmen-tation processes occur simultaneously during the degradation of polybenzoxazines: the cleavages of CN and CC bonds. The degradation mechanism is given in Scheme 6. Depending on the chemical nature of the R group and the substituent at the nitrogen atom, one of these two processes can predominate over the other. The study on the aliphatic amine-based polybenzoxazine model dimers showed that the size of the amines has a significant effect
on the type of cleavage [18]. The benzoxazine dimers with small amines tend to undergo the CC cleavage and form monomers, confirming the occurrence of a reverse Mannich reaction, while the dimers with large amines or aromatic amines tend to favor the CN cleavage, which subse-quently causes the formation of Schiff bases.
In brief, the thermal degradation products of polyben-zoxazines can be grouped into eight different types of com-pounds and these comcom-pounds can be grouped further into two categories. The first category comprises the primary decomposition products such as benzene derivatives, amines, phenolic compounds, and Mannich base com-pounds. These compounds are directly obtained from the degradation of the polybenzoxazines itself. The degradation mechanisms involve CN and CC bond cleavages, as discussed above. The proposed network structure of poly (BA-a) and the types of degradation products are illustrated inScheme 7. C--C cleavage C--N cleavage OH OH OH OH OH N R OH OH OH OH R N O OH OH • • • • N OH OH OH N R OH OH NH R + + R N R N R OH OH N R R N O
SCHEME 6 Thermal degradation of polybenzoxa-zine: the cleavages of CN and CC bonds.
OH OH N N N OH HO N OH N OH N HO OH OH N N N OH HO N OH N OH N HO SCHEME 5 Proposed structure of poly(Ph-a).
2.4. DP-MS Analysis of Polybenzoxazines
In the DP-MS technique, mass spectra are recorded contin-uously during the pyrolysis of the sample at a selected heat-ing rate. The total ion current (the variation of total ion yield as a function of temperature), TIC, curve gives information on thermal characteristics such as thermal stability and temperature ranges where the weight losses occurred, as in the case of the TGA analyses. The presence of more than one peak in the TIC curve points to either a multistep thermal degradation and/or the presence of more than one component with different thermal stabilities. Analyses of pyrolysis mass spectra allow the identification of thermal
degradation products. However, the pyrolysis mass spectra of polymers are usually very complex as the thermal deg-radation products further dissociate in the mass spectrom-eter during ionization. In addition, all the fragments with the same mass to charge ratio contribute to the intensity of the same peak in the mass spectrum. Thus, in pyrolysis MS analysis, not only the detection of a peak but also the variation of its intensity (single ion pyrograms, evolution profiles) as a function of temperature has particular importance.
The thermal decomposition processes in polybenzoxa-zines were studied by the application of various tech-niques [12–19]. Previous work concentrated on the use of Primary decomposition products
Benzene derivatives Amines Phenolic compounds Mannich base compounds
Secondary decomposition products
Biphenyl compounds
2,3 benzofuran
derivatives Isoquinoline derivatives Phenanthridine derivatives
Char formation OH OH OH OH N N N N OH OH O OH OH N NH NH N N OH OH OH OH N N N N OH OH O OH OH N NH2 NH2 N N
TGA-FTIR [12,13]. With the use of the TGA-GC-MS tech-nique, the identification of volatile products of polybenzox-azine model dimers and oligomers to investigate the thermal degradation mechanisms of polybenzoxazines was proven to be effective [18–21]. Recently, DP-MS analyses of polybenzoxazine based on phenol and methyl amine [22], and aromatic amine-based polynaphthoxazine [25], and polymers involving benzoxazine moieties along the main chain [26] were performed to gain a better under-standing of thermal decomposition characteristics. Existence of chains with different structures was suggested as a conse-quence of the presence of units with varying thermal stabilities.
2.4.1. Thermal Decomposition of Aliphatic and
Aromatic Amine-Based Polybenzoxazines
DP-MS analyses of typical aliphatic and aromatic amine-based polybenzoxazine analogs, poly(Ph-m), poly(BA-m), poly(Ph-a), and poly(BA-a), were studied to investi-gate the effect of amine groups on thermal characteristics. The curing programs applied were determined by DSC measurements as 0.5 hr at 160, 180, and 200C and 1.5 h at 210C for poly(Ph-m), poly(BA-m), and poly(BA-a), and 0.5 h at 160, 170, and 180C and 1.5 h at 190C for poly(Ph-a). The total ion current, TIC, curves recorded dur-ing the pyrolysis of the polymer samples were quite similar to the corresponding derivative mass loss curves (Figures 5 and6). The TGA data indicated higher char yields for phe-nol-based polybenzoxazines, more than 60% for methylamine, about 50% for aniline-based polybenzoxazines, and only about 40% for the corresponding bisphenol A-based polybenzoxazines. The analysis of pyrolysis mass spectra indicated the evolution of almost similar products for both phenol and bisphenol A-based polyben-zoxazines. The differences in the spectra were mainly due to the type of the amine unit present: methyl amine or aniline.
DP-MS analyses revealed that thermal degradation of polybenzoxazines starts with the loss of alkyl amines and diamines in the case of poly(Ph-m) and poly(BA-m), and with the evolution of aniline in the case of poly(Ph-a) at around 280 C. Single ion pyrograms of CH2NCH2
(42 Da) for poly(Ph-m) and poly(BA-m) and those of C5H5NH2 (93 Da) for poly(Ph-a) and poly(BA-a) are
shown inFigure 7as representative examples. In the case of poly(BA-a), the evolution of aniline was detected at higher and broader temperature ranges. The low tempera-ture loss of N-containing fragments was associated with the decomposition of the units generated by the coupling of CH3NCH2 or (C6H5)NCH2 groups generated by the
cleavage of the labile oxazine ring during thermal polymerization. As the evolution of methyl amines and aniline were also detected during the curing process, it was proposed that for poly(BA-a), this step of thermal decomposition was most probably completed during the polymerization of the monomer, BA-a. Emission of alkyl amines was almost totally completed in the first step of the thermal decomposition of methylamine-based polyben-zoxazines. On the other hand, aniline evolution was
100 100 b. TIC curve b. TIC curve a. Poly-(Ph-m) b. Poly-(BA-m) 200 300 280 °C 285 °C 420 °C 475 °C Temperature (°C) Temperature (°C) 400 500 600 200 300 400 500 600 80 60 40 20 0 200 400 a. TGA curve 420 °C 271 °C 503 °C 268 °C Temperature (°C) Temperature (°C) 600 60 80 100 a.TGA curve 0 200 400 600 800 0.00 0.05 0.10 0.15 0.20 –02 0.0 Deri v. W ei g ht (%/°C ) Deri v. W eight (%/°C) W eight (%) W eight (%) 0.2 0.4 0.6
detected over a broad temperature range during the pyrolysis of poly(Ph-a) and poly(BA-a).
Evolution profiles of fragments involving aromatic units showed several peaks and shoulders, revealing the presence of units with different thermal stabilities and hence, the presence of linkages with different structures along
the polymer chains. Thermal degradation of the chains generated by the attack of NCH2groups atortho and para
positions of the phenol ring were expected to occur by random cleavages at b carbon to phenol or N atom, as in case of the EI dissociation of the monomer and dimer. Thus, the fragments such as HOC6H4CH2NCH3 (136 Da) and 200 ´1.2 a. CH2NCH2, 42 Da b. C6H5NH2, 93 Da 282 °C 385 °C Poly(BA-m) Poly(BA-a) 400 600 200 ´3.6 ´1.0 ´1.0 285 °C Poly(Ph-m) 400 Temperature (°C) Temperature (°C) 600 200 400 600 270 °C Poly(Ph-a) 200 400 600
FIGURE 7 Single ion pyrograms of (a) CH2NCH2and (b) aniline detected during pyrolysis of phenol and bisphenol A-based polybenzoxazines.
100
200 b. TIC curve b. TIC curve
I. Poly-(Ph-a) II. Poly-(BA-a)
400 405 °C 480 °C 506 °C 431 °C 275 °C Temperature (°C) Temperature (°C) 600 200 400 600 80 60 40 a. TGA curve 472 °C 411 °C 406 °C 468 °C 269 °C Temperature (°C) Temperature (°C) 0 200 400 600 0 60 80 100 200 a. TGA curve 400 600 0.0 0.2 0.4 0.6 0.8 –0.1 0.0 Deri v. W ei g ht (%/°C ) Deri v. W eight (%/°C) W eight (%) W eight (%) 0.1 0.2 0.3
HOC6H4CH2N (150 Da) reaching maximum yield
at around 474 and 420 C for poly(Ph-m) and poly (BA-m), respectively, and the corresponding fragments HOC6H4CH2NC6H5 (198 Da) and HOC6H4CH2N(C6H5)
CH2 (212 Da) reaching maximum yield at around 420
and 385C for poly(Ph-a) and poly(BA-a), respectively, were associated with the decomposition of these units. This proposal led to the conclusion that the thermal stability of the chains generated by the attack of NCH2groups atortho
andpara positions of the phenol ring increased in the order poly(BA-a) < poly(BA-m) < poly(Ph-a) < Poly(Ph-m). InFigures 8and9, the evolution profiles of C6H5CH2
(91 Da), C6H5OH (94 Da), and HOC6H4CH2(107 Da) are
shown. Single ion pyrograms of HOC6H4CH2NCH3
(136 Da) for methylamine-based polybenzoxazines and HOC6H4CH2N(C6H5) (198 Da) for aniline-based
polyben-zoxazines are also presented in the figures. The yield of phenyl involving fragments, in the temperature region where the evolution of N involving fragments was dominant, was significantly low. These findings were explained through the generation of unsaturated units by coupling of the radicals produced upon the loss of amine or aniline units. As an example, single ion pyrograms of CH2C6H4CH¼¼CHC6H4CH2 (206 Da) are included in
Figures 8and9. Again, an increase was detected in the ther-mal stability of the units involving vinylene linkage in the order poly(BA-a) < poly(BA-m) < poly(Ph-a) < poly (Ph-m).
The loss of products, especially during the pyrolysis of poly(Ph-m) and poly(Ph-a) at elevated temperatures, above 500 C, was associated with the degradation of unsaturated, most probably cross-linked, polymer chains. In the case of poly(Ph-m), the evolution of thermal decomposition products continued at higher temperatures in accordance with the relatively high char yield. On the other hand, for bisphenol A-based polybenzoxazines, the yield of decomposition products in the high temperature ranges was noticeably lower, pointing to a limited extent of cross-linking compared to phenol-based analogs.
The existence of peaks representing additional products involving aromatic units in the pyrolysis mass spectra of aniline-based polybenzoxazines, poly(Ph-a) and poly (BA-a), was explained by the decomposition of the chains generated by the attack of NCH2groups atortho and para
positions of the aniline ring [18]. These products were max-imized at relatively low temperatures, at around 392 and 370 C, for poly(Ph-a) and poly(BA-a), respectively, pointing to a lower stability compared to the chains gener-ated by the attack of NCH2 groups at ortho and para
positions of the phenol ring (Figure 10).
To sum up, DP-MS analysis indicated that thermal decomposition of alkyl amine and aryl amine-based poly-benzoxazines starts with the loss of alkyl amines and/or aryl amines. In general, the thermal decomposition of phenol-based polybenzoxazines is completed at higher tempera-tures compared to the corresponding bisphenol A-based
300 ´1.1 ´4.3 ´1.0 ´59 ´6.8 474 °C 420 °C 515 °C 430 °C 520 °C 570 °C a. Poly(Ph-m) b. Poly(BA-m) ´1.1 C7H7, 91 Da HOC6H5, 94 Da HOC6H4CH2, 107 Da HOC6H4CH2N, 136 Da C6H4CH=CHC6H4CH2N or CH2C6H4CH=CHC6H4CH2, 206 Da ´3.7 ´1.0 ´2.3 ´1.4 500 Temperature (°C) 300 500 Temperature (°C)
FIGURE 8 Single ion pyrograms of some representative fragments involving aromatic structures detected during the pyrolysis of (a) poly(Ph-m) and (b) poly(BA-m).
analogs. This may be due to the presence of relatively weak C(CH3)2linkages joining the two phenyl groups. The
pres-ence of very intense peaks due to fragments involving only one phenyl ring pointed to the cleavage of these linkages readily decreasing the overall thermal stability of the bisphenol A-based polybenzoxazines.
2.4.2. Thermal Decomposition of
Furfurylamine-Based Polybenzoxazines
DP-MS analysis of furfurylamine-based polybenzoxazines poly(Ph-f) and poly(BA-f) were performed in order to investigate the effect that a polymerizable group substituted
to the amine group had on thermal characteristics. InFigure 11, the structures of the benzoxazine monomers thermally polymerized to prepare furfurylamine-based polybenzoxazines are given.
The polybenzoxazines based on furfurylamine and phenol or bisphenol A were prepared by applying two different curing programs. For poly(Ph-f1) and poly
(BA-f1), the last step of the stepwise curing was 190C,
and for poly(Ph-f2) and poly(BA-f2), the last step of the
stepwise curing was 240C. The TIC curves obtained dur-ing the pyrolysis of poly(Ph-f1) and poly(Ph-f2) are shown
inFigure 12. The relative intensity of the low temperature peak, which was maximum at 280 C, decreased upon
300 400 Temperature (°C) 500 ´26.9 ´10.0 ´6.0 ´28.6 392 °C 370 °C
I. poly(Ph-a) II. poly(BA-a)
300 C6H5NHC2H2NHC6H5 and/or C6H4NHCH2C6H4NHCH2, 210 Da HOC6H3CH=CHC6H4 and/or C6H4NCH2C6H4N 194 Da 400 Temperature (°C) 500
FIGURE 10 Evolution of fragments indicating decomposition of chains generated by attack of NCH2groups atortho and para positions of aniline ring.
´9.5 ´6.6 ´2.3 ´31.0 ´32.4 410 °C 390 °C 508 °C 460 °C 420 °C 435 °C 500 °C 532 °C a. Poly(Ph-a) b. Poly(BA-a) ´3.2 C7H7, 91 Da HOC6H5, 94 Da HOC6H4CH2, 107 Da HOC6H4CH2N (C6H5), 198 Da C6H4CH=CHC6H4CH2N or CH2C6H4CH=CHC6H4CH2, 206 Da ´3.6 ´2.2 ´9.7 ´12.2 300 400 500 Temperature (°C) 300 400 500 Temperature (°C)
FIGURE 9 Single ion pyrograms of some representative fragments involving aromatic structures detected during the pyrolysis of (a) poly(Ph-a) and (b) poly(BA-a).
curing at 240C. Furthermore, the broad peak, which was maximum at around 467C, became narrower.
The pyrolysis mass spectra of poly(Ph-f1) recorded
around 280 C were dominated with peaks due to CH2NHCH2C4H3O (110 Da), CH2C4H3O (81 Da), and
C4H3O (67 Da). These products also showed a high
temperature peak with a maximum at 461C in their single ion pyrograms (Figure 13a). The evolution of the pyrrole dimer (C4H2O)2(132 Da) also confirmed the polymerization
through the pyrrole units. Peaks due to fragments involving the phenyl ring appeared in the pyrolysis mass spectra recorded above 400 C. These decomposition products showed almost similar evolution profiles unlike what was observed for poly(Ph-m) and poly(Ph-a). The only differ-ence was the existdiffer-ence of a high temperature shoulder at around 555C in some of the evolution profiles of phenyl in-volving fragments such as C6H5OH (94 Da) and C6H5CH2
(91 Da) that were associated with the decomposition of cross-linked units, as in the case of poly(Ph-m).
For the sample cured at 240 C, the low temperature peak was not present in the evolution profiles of furfurylamine involving fragments, indicating that this step
of thermal decomposition was completed during the curing process (Figure 13b). Besides the disappearance of peaks at around 280C, the trends in the evolution profiles of all the decomposition products were almost identical with the corresponding ones for poly(Ph-f1), indicating that once
the furfurylamine fragments were lost, similar polymer structures were produced.
The thermal decomposition of the corresponding bis-phenol A-based polybenzoxazines, poly(BA-f1) and poly
(BA-f2) followed almost similar paths. Yet, the thermal
stability of poly(BA-f1) and poly(BA-f2) was slightly
decreased compared to that of phenol-based analogs (Figure 14). Again, the low temperature peak in the evolution profiles of furfurylamine involving fragments, with a maximum at 275C for poly(BA-f1), disappeared
in the evolution profiles of the corresponding decomposition products of the poly(BA-f2) sample cured at 240 C
(Figure 15). Upon curing at higher temperatures, the peaks in the evolution profiles of phenyl involving fragments sharpened, indicating the generation of a more homogeneous structure. Yet, the relative intensity of the high temperature shoulders also decreased, revealing a decrease in the extent of cross-linking as well.
2.4.3. Thermal Decomposition of Polysiloxanes
and Polyetheresters Containing Benzoxazine
Moieties Along the Main Chain
2.4.3.1. Poly(B-ala-co-Tetramethyldisiloxane) (PBTMDS) and Poly(B-ala-co-Dimethylsiloxane) (PBDMS)
The structures of poly(B-ala-co-tetramethyldisiloxane) (PBTMDS) and poly(B-ala-co-dimethylsiloxane) (PBDMS) are given inFigure 16. The thermal degradation products of polysiloxane-containing benzoxazine units along the main chain can be grouped into three categories: alkyl amines, siloxanes, and fragments involving the aromatic ring [26]. In Figure 17, the evolution profiles of CH2NH2
CH3 CH3 O N O N O O N O O CH3 CH3 O N O N O O N O O CH CH3 O N O N O O N O O CH3 CH O N O N O O N O O Ph-f BA-f
FIGURE 11 Structure of benzoxazine monomers used to prepare furfur-ylamine-based polybenzoxazines. Temperature (°C) 200 400 600 Temperature (°C) 200 400 600 I. Poly(Ph-f1) II. Poly(Ph-f2) 467 °C 467 °C 280 °C
(30 Da), CH2NCH2 (42 Da) and CH2NCH2CH2 or
HNCH2CH¼¼CH2 (56 Da), HSi(CH3)2[OSi(CH3)2]2
(207 Da), HSi(CH3)2[OSi(CH3)2]4(355 Da), C6H5(77 Da),
C6H5OH (94 Da), C6H5CH2NH2 and/or HOC6H4CH2
(107 Da), and C6H4C2H2C6H4CH2 and/or HSi(CH3)[OSi
(CH3)2]2(192 Da) are shown. The presence of intense peaks
of low mass oligosiloxanes was associated with unwanted polymerization reactions during the synthesis.
The thermal degradation of both PBTMDS and PBDMS was initiated by the evolution of alkyl amines, such as CH2NH2(30 Da), CH2NCH2(42 Da), and CH2NCH2CH2
and/or HNCH2CH¼¼CH2 (56 Da), just above 250 C, 200
´8.8 ´38.3 ´70.5
I. Poly(Ph-f1) II. Poly(Ph-f2)
280 °C 455 °C 450 °C 470 °C 466 °C 463 °C 555 °C 559 °C 415 °C 400 Temperature (°C) 600 200 CH2C4H3O81 Da CH2NHCH2C4H3O 110 Da C6H5CH2 91 Da C6H5OH 94 Da HOC6H4CH2 107 Da CH2NHCH2(C4H2O)2 175 Da (C4H2O)2 132 Da ´6.8 ´7.7 ´2.0 ´2.5 ´2.0 ´2.5 ´3.2 ´30.7 ´3.2 320 °C 416 °C 400 Temperature (°C) 600
FIGURE 13 Single ion pyrograms of some representative fragments detected during the pyrolysis of (a) poly(Ph-f1) and (b) poly(Ph-f2).
I. Poly(BA-f1) II. Poly(BA-f2)
Temperature (°C) 200 400 600 463 °C 449 °C 297 °C Temperature (°C) 200 400 600
indicating the ready decomposition of the relatively labile oxazine ring. The yield of fragments involving siloxane and phenyl units was extremely low in the region where al-kyl amine evolutions were detected. At around 420C, the
evolution of alkyl amines, siloxanes, and fragments involv-ing the aromatic rinvolv-ing followed almost similar trends. Hence, thermal decomposition via random cleavages at relatively labile SiC bonds was proposed at around 420 C. No 200
´2.7 ´13.2 ´8.0
I. Poly(BA-f1) II. Poly(BA-f2)
275 °C 420 °C 445 °C 550 °C 447 °C 530 °C 300 °C 460 °C 425 °C 400 Temperature °C 600 200 CH2C4H3O 81 Da CH2NHCH2C4H3O 110 Da C6H5OH 94 Da C6H5CH2 91 Da HOC6H4CH2 107 Da CH2NHCH2(C4H2O)2 175 Da (C4H2O)2 132 Da ´9.6 ´1.9 ´1.1 ´1.1 ´1.5 ´1.8 ´6.4 ´30.0 ´35.0 325 °C 400 Temperature °C 600
FIGURE 15 Single ion pyrograms of some representative fragments detected during the pyrolysis of (a) poly(BA-f1) and (b) poly(PBA-f2).
O O N N Si O Si n PBTMDS O O N N Si O Si O m Si n PBDMS
FIGURE 16 The structures of poly(B-ala-co-tetramethyldi-siloxane) (PBTMDS) and poly(B-ala-co-dimethylpoly(B-ala-co-tetramethyldi-siloxane) (PBDMS).
other similarity existed in the evolution profiles of these three groups of products. The loss of fragments involving aromatic units, such as C6H5OH and C6H4C2H2C6H4CH2,
was detected at high temperatures where the evolution of siloxane fragments had almost totally been completed. Thus, generation of polybenzoxazine chains with unsatu-rated units and/or cross-linked structure upon cleavage of the oxazine ring was proposed. These units decomposed at around 550 and 605C, in the temperature range higher than that detected for the similar products during the ther-mal degradation of polybenzoxazines based on bisphenol A and methylamine (510 and 550C). These findings lead to the conclusion that an increase in thermal stability and in the extent of cross-linking occurred when the benzoxazine moieties were separated by siloxane units.
In the case of PBDMS, the relative yields of both alkyl amines and fragments involving aromatic units were signifi-cantly low as expected, as each bisphenol A unit was separated by polysiloxane chains. Three overlapping peaks with max-ima at 285, 390, and 500C were detected in the evolution pro-files of almost all the thermal degradation products of PBDMS and were associated with degradation via random cleavages at relatively labile SiC bonds (Figure 17). The broad evolu-tion profiles with several overlapping peaks were associated with the polydispersity of the sample and the presence of low mass oligomers.
Two noticeable differences were the low temperature loss of alkyl amines and the high temperature evolutions of fragments involving aromatic units. Weak peaks with maxima at 555 and 605 C were also observed in the 200 ´2.2 ´2.3 ´4.0 I. PBTMDS II. PBDMS 278 °C 420 °C 510 °C 550 °C 605 °C 285 °C390 °C 500 °C 555 °C 375 °C 398 °C 300 °C 400 Temperature (°C) 600 200 CH2NH2 30 Da CH2=NCH2 42 Da C6H5 77 Da C6H5 OH 94 Da HOC6H4CH2 107 Da CH2=NCH2CH2 56 Da HSi(CH3)2[OSi(CH3)2]2 207 Da HSi(CH3)2[OSi(CH3)2]4 355 Da ´3.2 ´15.1 ´1.7 ´8.6 ´1.8 ´7.2 ´19.8 ´82.3 ´70.2 ´5.8 ´6.8 ´21.0 250°C 400 Temperature (°C) 600 C6H4C2H2C6H4CH2 and/or HSi(CH3)[OSi(CH3)2]2 192 Da
evolution profiles of fragments involving aromatic units, yet their relative intensities were significantly weak compared to what was recorded for PBTMDS. Hence, it can be concluded that as siloxane decomposition took place, loss of bisphenol A units also took place and the extent of polymerization by the opening of the oxazine ring and cross-linking was significantly lowered when longer polysiloxanes were used instead of oligosiloxanes.
2.4.3.2. Polyetherester Containing Benzoxazine Moiety (PEE-BT)
The structure of polyetherester-containing benzoxazine moiety (PEE-BT) is given inFigure 18. Evolution of alkyl amines, such as CH2NH2 (30 Da) and CH2¼NCH2CH2
(56 Da), was also detected at the early stages of pyrolysis of PEE-BT, at around 278 and 297C (Figure 19). These
O O N N O O O O R O n O (PEE-BT) FIGURE 18 The structure of polyether
ester-containing benzoxazine moiety (PEE-BT).
100 300 Temperature (°C) 500 ´2.6 ´2.4 ´1.7 ´18.4 ´4.6 ´6.5 ´14.3 260 °C 275 °C 297 °C 425 °C 360 °C 315 °C 455 °C 550 °C CH2 = NH2 30 Da CH2 = NCH2 42 Da CH2 = NCH2CH2 56 Da C6H5 77 Da C6H5CH2 91 Da C6H5OH 94 Da HOOCC6H4CO 149 Da HOC6H4CH2 107 Da C2H4OOCC6H4CO2 192 Da FIGURE 19 Single ion pyrograms of some representative
values are very close to the corresponding values recorded for poly(Ph-m), poly(BA-m), PBTMDS, and PBDMS. It is clear that the decomposition of the oxazine ring was not affected by the nature of the chains connecting the bisphenol A units or by the unit substituted on N.
Decomposition of terephthaloyl groups was detected at around 315 and 360C. The yield of products generated by the cleavage ofCH2OC¼¼OR bonds was maximized
at lower temperatures compared to those generated by the cleavage ofCH2OCH2 groups. As in the case for
PBTMDS, the products involving aromatic units were maxi-mized at around 425C. The shoulder in the evolution profiles of these products at around 315C was attributed to the deg-radation of etherester groups yielding the fragments with the same m/z values, i.e., C6H5CO and C6H4CH2NH (105 Da).
These products showed high temperature tails in their evolution profiles. Products that can only be generated by degradation of bisphenol A units showed a maximum at 455C and a high temperature shoulder around 550C, at noticeably lower temperatures than the corresponding values detected for PBTMDS. The lower thermal stability was associated with the lower thermal stability of the polyether es-ter chains compared to polysiloxanes. Hence, it was con-cluded that thermal stability and extent of cross-linking enhanced when the benzoxazine moieties were separated by thermally more stable units such as siloxanes. However, when the siloxane chain units were long, the possibility of polybenzoxazine growth decreased significantly.
3. CONCLUSIONS
Thermal decomposition of aliphatic amine-based poly-benzoxazines starts with the loss of CH2NCH2R groups
by cleavage at b carbon to N atom at around 280 C. The identity of the R group does not have any significant effect on the loss of alkyl amines in accordance with expectations as dependence of CH2N(CH2R)CH2bond
energy on the R group should be negligible. Polymers in-volving benzoxazine moieties connect to N of the oxazine rings by methylene, and CH2 groups also lose
CH2NCH2 groups following the cleavage of oxazine
rings. On the other hand, evolution of aniline is the first step of thermal degradation in the case of aniline-based polybenzoxazine.
Decomposition of products involving phenyl groups evolved above 400 C over a broad temperature range, indicating the presence of units with different thermal stabilities and, hence, structures. Thermal degradation of the chains generated by the attack of CH2(RCH2)NCH2
groups on ortho and para positions of the phenyl ring occurs at around 400 C. The thermal stability of these chains is lower than that of the chains involving vinylene linkages. Vinylene linkages combining two phenoyl or
bisphenoyl groups are produced by coupling reactions upon cleavage of the oxazine ring during curing or upon the loss of alkyl amines or aniline during thermal degradation. These units can further cross-link and increase the char yield during heating.
Although the R group substituted to NCH2 does not
have any significant effect on cleavage at b carbon to N atom, it controls the efficiency of coupling reactions. The presence of a bulky group decreases the probability of coupling reactions during the polymerization and/or thermal decomposition, thus decreasing the extent of cross-linking. The high char yield of methylamine-based polybenzoxazines supports these proposals. On the other hand, the presence of a polymerizable R group increases the thermal stability of the chains generated by the attack of RCH2NCH2 groups on ortho and para positions of
the phenyl ring.
In general, the thermal decomposition of phenol-based polybenzoxazines is completed at higher temperatures than the corresponding bisphenol A-based analogs. This may be due to the presence of relatively weak C(CH3)2linkages
joining the two phenyl groups. Successful attempts at the polymerization of benzoxazine moieties along poly-mer chains increased thermal stability and the extent of cross-linking. The increase in thermal stability depends on the stability of the polymer chains separating the benzoxazine moieties. However, when the polymer chain units are long, the possibility of polybenzoxazine growth decreases significantly.
REFERENCES
[1] C.A. Wilkie, TGA/FTIR: an extremely useful technique for studying polymer degradation, Polym. Degrad. Stabil. 66 (3) (1999) 301–306. [2] F. Hoffmann, R. Riesen, J. Foreman, Characterization of thermal stability and reaction products by means of TGA-FTIR coupling, Am. Lab. 32 (1) (2000) 13.
[3] K.G.H. Raemaekers, J.C.J. Bart, Applications of simultaneous ther-mogravimetry-mass spectrometry in polymer analysis, Thermochim. Acta 295 (1997) 1–58.
[4] K.L. Sobeih, M. Baron, J. Gonzalez-Rodriguez, Recent trends and developments in pyrolysis-gas chromatography, J. Chromatog. A 1186 (2008) 51–66.
[5] S. Tsuge, H. Ohtani, Structural characterization of polymeric mate-rials by pyrolysis-GC/MS, Polym. Degrad. Stabil. 58 (1-2) (1997) 109–130.
[6] M. Blazso´, Recent trends in analytical and applied pyrolysis of polymers, J. Anal. Appl. Pyrol. 39 (1997) 1–25.
[7] M. Herrera, G. Matuschek, A. Kettrup, Fast identification of polymer additives by pyrolysis-gas chromatography/mass spectrometry, J. Anal. Appl. Pyrol. 70 (1) (2003) 35–42.
[8] M. Blazso´, In situ modification of pyrolysis products of macromole-cules in an analytical pyrolyser, J. Anal. Appl. Pyrol. 74 (1-2) (2005) 344–435.
[9] Z. Parsi, T. Go´recki, J. Poerschmann, Advances in non-discriminat-ing pyrolysis, J. Anal. Appl. Pyrol. 74 (2005) 11–18.
[10] K. Qian, W.E. Killinger, M. Casey, Rapid polymer identification by in-source direct pyrolysis mass spectrometry and library searching techniques, Anal. Chem. 68 (1996) 1019–1027.
[11] G. Montaudo, Direct mass spectrometry of polymers, Br. Polym. J. 18 (4) (1986) 231–235.
[12] H.Y. Low, H. Ishida, Mechanistic study on the thermal decomposi-tion of polybenzoxazines: effects of aliphatic amines, J. Polym. Sci. B Polym. Phys. 36 (1998) 1935–1946.
[13] H.Y. Low, H. Ishida, Structural effects of phenols on the thermal and thermo-oxidative degradation of polybenzoxazines, Polymer 40 (15) (1999) 4365–4376.
[14] H.Y. Low, H. Ishida, An investigation of the thermal and thermo-oxidative degradation of polybenzoxazines with a reactive functional group, J. Polym. Sci. B Polym. Phys. 37 (1999) 647–659. [15] Z. Brunovska, H. Ishida, Thermal study on the copolymers of
phtha-lonitrile and phenylnitrile-functional benzoxazines, J. Appl. Polym. Sci. 73 (1999) 2937–2949.
[16] H.J. Kim, Z. Brunovska, H. Ishida, Synthesis and thermal charac-terization of polybenzoxazines based on acetylene-functional monomers, Polymer 40 (23) (1999) 6565–6573.
[17] H. Ishida, D.P. Sanders, Improved thermal and mechanical properties of polybenzoxazines based on alkyl-substituted aromatic amines, J. Polym. Sci. B Polym. Phys. 38 (2000) 3289–3301.
[18] K. Hemvichian, A. Laobuthee, S. Chirachanchai, H. Ishida, Thermal decomposition processes in polybenzoxazine model dimers investi-gated by TGA-FTIR and GC-MS, Polym. Degrad. Stabil. 76 (2002) 1–15.
[19] H.Y. Low, H. Ishida, Improved thermal stability of polybenzoxazines by transition metals, Polym. Degrad. Stabil. 91 (2006) 805–815. [20] K. Hemvichian, H. Ishida, Thermal decomposition processes in
aromatic amine-based polybenzoxazines investigated by TGA and GC-MS, Polymer 43 (2002) 4391–4402.
[21] K. Hemvichian, H.D. Kim, H. Ishida, Identification of volatile products and determination of thermal degradation mechanisms of polybenzoxazine model oligomers by GC-MS, Polym. Degrad. Stabil. 87 (2005) 213–224.
[22] S. Bagherifam, T. Uyar, H. Ishida, J. Hacaloglu, The use of pyrolysis mass spectrometry to investigate polymerization and degradation processes of methyl amine-based benzoxazine, Polym. Test. 29 (4) (2010) 520–526.
[23] H.D. Kim, H. Ishida, A study on hydrogen bonding in controlled-structure benzoxazine model oligomers, Macromol. Symp. 195 (2003) 123–140.
[24] L. Pauling, Chemical Bonds, in: Cornell University Press, New York, 1960, p. 85.
[25] T. Uyar, Z. Koyuncu, H. Ishida, J. Hacaloglu, Polymerisation and degradation of an aromatic amine-based naphthoxazine, Polym. Degrad. Stabil. 93 (12) (2008) 2096–2103.
[26] S. Bagherifam, B. Kiskan, B. Aydogan, Y. Yagci, J. Hacaloglu, Thermal degradation of polysiloxane and polyetherester containing benzoxazine moieties in the main chain. J. Anal. Appl. Pyrol. 90 (2011) 155–163.
